Dynamo
A dynamo is an
Today, the simpler
History
Induction with permanent magnets
The operating principle of electromagnetic generators was discovered in the years 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.
He also built the first electromagnetic generator, called the
This design was inefficient, due to self-cancelling counterflows of current in regions of the disk that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Faraday and others found that higher, more useful voltages could be produced by winding multiple turns of wire into a coil. Wire windings can conveniently produce any voltage desired by changing the number of turns, so they have been a feature of all subsequent generator designs, requiring the invention of the commutator to produce direct current.
First dynamos
The first commutated dynamo was built in 1832 by
Pixii found that the spinning magnet produced a pulse of current in the wire each time a pole passed the coil. However, the north and south poles of the magnet induced currents in opposite directions. To convert the alternating current to DC, Pixii invented a commutator, a split metal cylinder on the shaft, with two springy metal contacts that pressed against it.
This early design had a problem: the electric current it produced consisted of a series of "spikes" or pulses of current separated by none at all, resulting in a low average power output. As with electric motors of the period, the designers did not fully realize the seriously detrimental effects of large air gaps in the magnetic circuit.
Antonio Pacinotti, an Italian physics professor, solved this problem around 1860 by replacing the spinning two-pole axial coil with a multi-pole toroidal one, which he created by wrapping an iron ring with a continuous winding, connected to the commutator at many equally spaced points around the ring; the commutator being divided into many segments. This meant that some part of the coil was continually passing by the magnets, smoothing out the current.[1]
The
Self excitation
In 1827, independently of Faraday, Hungarian inventor
Around 1856, six years before
Practical designs
The dynamo was the first electrical generator capable of delivering power for industry. The modern dynamo, fit for use in industrial applications, was invented independently by
The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field.[citation needed] Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel.[9] The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.
The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils.
Rotary converters
After dynamos and motors were found to allow easy conversion back and forth between mechanical or electrical power, they were combined in devices called
The rotary converter can directly convert, internally, any type of electric power into any other. This includes converting between direct current (DC) and alternating current (AC),
The technology of rotary converters was replaced in the early 20th century by
Limitations and decline
Direct current machines like dynamos and commutated DC motors have higher maintenance costs and power limitations than alternating current (AC) machines due to their use of the commutator. These disadvantages are:
- The sliding friction between the brushes and commutator consumes power, which can be significant in a low power dynamo.[citation needed]
- Due to friction, the brushes and copper commutator segments wear down, creating dust. Large commutated machines require regular replacement of brushes and occasional resurfacing of the commutator. Commutated machines cannot be used in low particulate or sealed applications or in equipment that must operate for long periods without maintenance.
- The resistanceof the sliding contact between brush and commutator causes a voltage drop called the "brush drop". This may be several volts, so it can cause large power losses in low voltage, high current machines (see the huge commutator of the 7 volt electroplating dynamo in the adjacent picture). Alternating current motors, which do not use commutators, are much more efficient.
- There is a limit to the maximum current density and voltage which can be switched with a commutator. Very large direct current machines, say, with megawatt power ratings, cannot be built with commutators. The largest motors and generators are all alternating-current machines.
- The switching action of the commutator causes sparking at the contacts, posing a fire hazard in explosive atmospheres, and generating electromagnetic interference.
Although direct current dynamos were the first source of electric power for industry, they had to be located close to the factories that used their power. Electricity could only be distributed over distances economically as alternating current (AC), through the use of the transformer. With the 1890s conversion of electric power systems to alternating current, during the 20th century dynamos were replaced by alternators, and are now almost obsolete.
Etymology
The word 'dynamo' (from the Greek word dynamis (δύναμις), meaning force or power) was originally another name for an
The original "dynamo principle" of Werner von Siemens referred only to the direct current generators which use exclusively the self-excitation (self-induction) principle to generate DC power. The earlier DC generators which used permanent magnets were not considered "dynamo electric machines".[16] The invention of the dynamo principle (self-induction) was a major technological leap over the old traditional permanent magnet based DC generators. The discovery of the dynamo principle made industrial scale electric power generation technically and economically feasible. After the invention of the alternator and that alternating current can be used as a power supply, the word dynamo became associated exclusively with the 'commutated direct current electric generator', while an AC electrical generator using either slip rings or rotor magnets would become known as an alternator.
A small electrical generator built into the hub of a bicycle wheel to power lights is called a hub dynamo, although these are invariably AC devices,[citation needed] and are actually magnetos.
Design
The electric dynamo uses rotating coils of wire and magnetic fields to convert mechanical rotation into a pulsing direct electric
Commutation
The commutator is needed to produce
Excitation
The earliest dynamos used
Self-excited direct current dynamos commonly have a combination of series and parallel (shunt) field windings, which are directly supplied power by the rotor through the commutator in a regenerative manner. They are started and operated in a manner similar to modern portable alternating current electric generators, which are not used with other generators on an electric grid.
There is a weak residual magnetic field that persists in the metal frame of the device when it is not operating, which has been imprinted onto the metal by the field windings. The dynamo begins rotating while not connected to an external load. The residual magnetic field induces a very small electrical current into the rotor windings as they begin to rotate. Without an external load attached, this small current is then fully supplied to the field windings, which in combination with the residual field, cause the rotor to produce more current. In this manner, the self-exciting dynamo builds up its internal magnetic fields until it reaches its normal operating voltage. When it is able to produce sufficient current to sustain both its internal fields and an external load, it is ready to be used.
A self-excited dynamo with insufficient residual magnetic field in the metal frame will not be able to produce any current in the rotor, regardless of what speed the rotor spins. This situation can also occur in modern self-excited portable generators, and is resolved for both types of generators in a similar manner, by applying a brief direct current battery charge to the output terminals of the stopped generator. The battery energizes the windings just enough to imprint the residual field, to enable building up the current. This is referred to as flashing the field.
Both types of self-excited generator, which have been attached to a large external load while it was stationary, will not be able to build up voltage even if the residual field is present. The load acts as an energy sink and continuously drains away the small rotor current produced by the residual field, preventing magnetic field buildup in the field coil.
Uses
Historic
Dynamos, usually driven by
Large industrial dynamos with series and parallel (shunt) windings can be difficult to use together in a power plant, unless either the rotor or field wiring or the mechanical drive systems are coupled together in certain special combinations.[19]
Dynamos were used in motor vehicles to generate electricity for battery charging. An early type was the third-brush dynamo. They have, again, been replaced by alternators.
Modern
Dynamos still have some uses in low power applications, particularly where low voltage DC is required, since an alternator with a semiconductor rectifier can be inefficient in these applications.
Hand
The generator used for bicycle lighting may be called a "dynamo" but these are almost always AC devices and so, strictly, would be called "alternators".
See also
References
- ^ Anthology of Italian Physics, entry for Antonio Pacinotti, from the website of the University of Pavia
- ^ Birmingham Museums trust catalogue, accession number: 1889S00044
- ISBN 0750301457.
- ISBN 9780852968956.
- .
- ISBN 0-9665734-2-0.
- ^ "Ányos Jedlik biography". Hungarian Patent Office. Archived from the original on 4 March 2010. Retrieved 10 May 2009.
- doi:10.1038/053516a0.
- ^ "On the augmentation of the power of a magnet by the reaction thereon of currents induced by the magnet itself". Proceedings of the Royal Society. February 14, 1867.
- ISBN 978-0-07-144146-9.
- ^ Thompson, Sylvanus P. (1888), Dynamo-electric machinery: a manual for students of electrotechnics. London: E. & F.N. Spon. p. 140.
- ^ Jeffrey La Favre. "The Brush Dynamo".
- ^ "The Brush Electric Light". Scientific American. 2 April 1881. Archived from the original on 11 January 2011.
- ^ Williams, L. Pearce, “Michael Faraday,” p. 296-298, Da Capo series, New York, N.Y. (1965).
- ^ "Experimental Researches in Electricity", Vol. 1, Series I (Nov. 1831); footnote for Art. 79, p. 23, 'Ampère's Inductive Results', Michael Faraday, D.C.L, F.R.S.; Reprinted From The Philosophical Transactions Of 1846-1852, with other Electrical Papers from the Proceedings of the Royal Institution and Philosophical Magazine, Richard Taylor and William Francis, Printers and Publishers to the University of London, Red Lion Court, Fleet Str., London, England (1855).
- ^ Volker Leiste: 1867 – Fundamental report on dynamo-electric principle before the Prussian Academy of Sciences siemens.com Archived 2017-09-01 at the Wayback Machine
- ^ a b Lockwood, Thomas D. (1883). Electricity, Magnetism, and Electric Telegraphy. D. Van Nostrand. pp. 76–77.
magneto-electric machine.
- ^ Schellen, Heinrich; Nathaniel S. Keith (1884). Magneto-Electric and Dynamo-Electric Machines, Vol. 1. D. Van Nostrand. p. 471., translated from German by Nathaniel Keith
- ^ Dynamo-Electric Machinery: A Manual for Students of Electrotechnics, by Silvanus P. Thompson, 1901, 8th American Edition, Ch. 31, Management of Dynamos, pp. 765-777, Free digital access from Google Books, Cite search method: "dynamo" "coupling" via Google Scholar
External links
- The Electrification of the World – Werner von Siemens and the Dynamoelectric Principle Archived 2020-09-20 at the Wayback Machine Siemens Historical Institute